1. Field of the Invention
The present invention relates to an optical scanning device provided on an electro-graphic image forming apparatus and an image forming apparatus including the optical scanning device.
2. Description of the Related Art
A conventional electro-graphic image forming apparatus scans a photoreceptor (i.e., an image bearing member) with a laser beam that an optical scanning device having a polygon mirror emits, which serves as a light source (i.e., a deflect and scan unit), and forms an electrostatic latent image on the photoreceptor. Then, the image forming apparatus develops the electrostatic latent image with a toner, and transfers and fixes the toner image on a recording medium, thereby forming an image on the recording medium.
In such an image forming apparatus, a displacement of a laser beam occurs as to its irradiation position due to an installation error and insufficiency of manufacturing accuracy of an optical element provided on the optical scanning device, torsion of a housing containing the optical element, thermal deformation owing to heat generation by a drive motor that drives a polygon mirror, thermal deformation which another unit in the main body causes as a heat source, and torsion when the photoreceptor is installed. These phenomena bend and incline a scanning line of the laser beam with which the photoreceptor is scanned, relative to an ideal scanning line. This bending and inclination of the actual scanning line relative to the ideal scanning line reduce image quality.
Particularly, in a color image forming apparatus that forms a scanning line on each of four photoreceptors corresponding to each color of cyan (C), magenta (M), yellow (Y), and black (B), the inclination and bending of the scanning line corresponding to the respective colors significantly influence the quality of the image. In other words, the scanning line of the respective colors is different in degree and direction of the bending and inclination respectively. If such bending and inclination of the scanning line is present, the scanning lines that need to be superimposed are not superimposed on each other. Thus, a color displacement occurs and the quality of image is reduced.
In order to solve such a problem, a technique is discussed in Japanese Patent Application Laid-Open No. 2006-30705 which rotates an optical element (hereinafter, referred to as a lens) having power to refract a laser beam in a sub scanning direction around a rotation shaft parallel to an optical axis of the lens to adjust an inclination of scanning line, which is one factor of a color displacement.
However, as described above, the installed lens shows a geometric tolerance and an installation error, and an optical scanning device has a geometric tolerance and housing torsion. Therefore, when the lens is rotated about the rotation shaft parallel to the optical axis to adjust the inclination of scanning line, a laser beam is not necessarily incident on a predetermined position. If the above-described inclination correction is performed while the incident position of the laser beam on the lens is not suitable, the shape of a spot of the laser beam that forms an image on a photoreceptor may not be uniform depending on the scanning position of the laser beam.
FIG. 11 illustrates a spot diameter distribution (vertical axis) with respect to the amount of displacement (horizontal axis) between a passing position of a laser beam and the lens generatrix in lenses including aspheric surface lenses. The spot diameter refers to a maximum size of a region in a main scanning direction and a sub scanning direction among regions where the level of the quantity of light relative to the peak quantity of light is 1/e2 (e is the bottom of a natural logarithm). In other words, the spot diameter refers to a diameter of a cross section in a portion where the quantity of light relative to the peak quantity of light of the laser beam is 1/e2 (approximately the quantity of 14% relative to the peak quantity) in a Gaussian distribution that indicates the strength of the laser beam.
Referring to FIG. 11, it is understood that as the amount of displacement from the lens generatrix in an incident position of the laser beam is increased, the spot diameter becomes significantly larger both in the main scanning direction and the sub scanning direction. Further, it can be seen that as the amount of displacement is increased, the spot shape collapses such that the spot is deformed and its shape is distorted, or the spot is rotated.
When the laser beam is incident on a position displaced from the lens generatrix, if the lens is rotated and the inclination correction of the scanning line is performed, the amount of displacement described above may be increased depending on the position of the main scanning direction. Thus, depending on the position of the main scanning direction, the above-described distortion of the spot shape and enlargement of the spot diameter may be facilitated.
If the above-described adverse effect appears, a profile of a latent image when a photoreceptor is exposed does not become the predetermined profile. Accordingly, this causes reduction of image quality such as deterioration in density uniformity and roughness, or characters are not reproduced properly.